Based on my experience, the best way is to schedule the snapshot is to do it in weekly base. Then, after it's done. Stop the schedule and mount the snapshot as a regular volume. Then copy the data to the volume and that's it... So it is very simple procedure.
For other snapshot time, you just run the schedule again.
It seems that you mixed up between snapshot and backup. Back is used to save the data till you decide to delete it manually -like archive- But snapshot is like a temporary backup. As it only saved till the second snapshot time starts.
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